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I just picked up a used air compressor. Its a American IMC two stage compressor. It has a 5 hp electric motor. The compressor date is 1992. I am going to give the compressor a clean up and get it ready for service. My question is how do you know if you need a mag starter? I am not 100 percent sure what they do. Do some electric motors come with internal mag starters? Any info would be great.

Does the motor have a over temp pop out button on the end opposite the pulley?

Some good pics will help.

Being a two stage, the motor probably is a true 5 hp and may very well need a starter. The reason for one is two fold, first, the pressure switch would have to be huge to carry the amperage of the motor, so instead, a small, low amp rated pressure switch pressure is used and it operates the coil of the motor starter (simply a solenoid operated multiple contact switch) and the second need for a starter is that is contains a thermal overload, in the form of a heat element that is sized to the motor, so that if the motor is overloaded (such as the pressure switch sticking and the pressure going way too high) the heat element would get hotter than normal and cause a set of contacts to open, killing the circuit to the motor and stopping it. It simulates the motor overheating.

Went to their site and I guess it really is. They have it wired through the pressure switch. Another reason not mentioned why you need a starter is to pull in all three legs in one shot when you have three phase.

That is a "true" 5hp motor rated for compressor duty, with all those capacitors it's an "energy efficient" unit that draws maybe 22 amps. These are common on light commercial compressors like this one. The pressure switch is rated to handle the full current draw of the motor without needing a mag relay. If ever you need to replace the pressure switch, make sure to get one rated for the current draw.

Yes! Generally most electric motors with multiple capacitors on it (the round humps or caps on top and side) do not need a magnetic starter. The capacitors do all the work. Just wire it up as found and use an appropriate size breaker.

Yes! Generally most electric motors with multiple capacitors on it (the round humps or caps on top and side) do not need a magnetic starter. The capacitors do all the work. Just wire it up as found and use an appropriate size breaker.

Thanks guys. As you can see it needs some love. I will start a thread on its restoration soon.